Track circuits are used in the railroad industry to detect the presence of a train in a block of track. An AC overlay track circuit includes a transmitter and a receiver, with the transmitter configured to transmit an AC signal through the track rails at one end of a block of track and the receiver connected to the rails at the other end of the block and configured to detect the signal. Other than the connection through the track rails, there is typically no connection between the transmitter and receiver for a block. When a train is present in a block of track monitored by a track circuit, the train shunts, or shorts, the two rails, with the result that no signal is received at the receiver. Thus, the receiver uses the presence or absence of a detected signal to indicate whether or not a train is present in the block. It is therefore very important that a receiver in a particular block of interest not interpret spurious signals or stray signals from a transmitter in another block of track or some other transmitter as originating from the transmitter associated with the block of interest.
In order to prevent a spurious signal detected at the receiver from being mistakenly interpreted as originating from the transmitter, transmitters are typically configured to transmit at one of a plurality of fixed frequencies, and personnel responsible for installing the track circuits ensure that all track circuits in close spatial proximity are configured to transmit on different frequencies. In order to provide further assurance that a detected signal originates from a corresponding transmitter, the signal is modulated by a code. In some track circuits, the modulation is performed using a binary frequency shift key technique. With this technique, frequencies above or below the nominal center frequency are transmitted to convey a bit of information (i.e., a frequency above the nominal center frequency represents a logical “1” while a frequency below the nominal center frequency represents a logical “0”). What is needed is a method of reliably detecting these codes. Because there is typically no connection between the transmitter and receiver for a track block, a non-coherent detection method must be used.